There have been some rare cases where valid devices have been flagged as invalid due to a discrepancy in device serial numbers. In this unlikely event, you may safely use your test by following the printed instructions in the box, although, you will not be able to use the app to store your result for later lookup or to report your result to public health authorities in your area.
If you can take a screenshot of the error page showing your serial number as invalid, please contact Celltrion support at celltrionusa.CS@celltrion.com, you may be eligible for a free replacement.